Do Equine Metabolic Diseases Affect Reproduction?

Having trouble getting your mare pregnant? It might be time to take a good look at her weight and metabolic condition. Some veterinarians now believe that metabolic diseases likely have a negative impact on equine reproduction.

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Having trouble getting your mare pregnant? It might be time to take a good look at her weight and metabolic condition. Some veterinarians now believe that metabolic diseases likely have a negative impact on equine reproduction.

At the 2013 Society for Theriogenology Conference, held Aug 7-10 in Louisville, Ky., Peter Morresey, BVSc, MACVSc, Dipl. ACT, ACVIM, presented a lecture on what we know about metabolic diseases and equine reproduction.

Knowledge from Other Species

At the moment, much of what veterinarians know about reproduction-related metabolic disorders in horses comes from human medicine and research in other species, Morresey explained. He touched on research first:

Polycystic ovary syndrome (PCOS) is a common endocrine disorder in women of child-bearing age, he said, and is characterized by numerous ovarian cysts, high male hormone levels, chronic anovulation (ovulation in which the ovaries do not release an egg), and other metabolic disturbances. Affected women have conditions such as obesity and insulin resistance—which has been characterized as the major risk factor related to PCOS. Additionally, he said, PCOS is associated with gestational diabetes development, and affected women tend to have increased insulin resistance compared to healthy pregnant women. Finally, women with PCOS have an increased risk of first trimester abortion compared to healthy women.

Research has shown that obesity in men can negatively affect sperm production, decrease testosterone levels, and suppressed testicular function, among other negativities.

Research in mice showed that females fed a high-fat diet for 16 weeks were less likely to ovulate than mice fed a normal diet. Additionally, the high-fat diet mice had fewer and smaller mature oocytes, and their fetuses developed at a slower rate. Finally, researchers found that the oocytes had decreased IGF-1 receptors—indicating insulin resistance at a cellular level—and the fetuses had more developmental defects than those conceived in healthy mice—indicating the high-fat diet permanently altered how the fetuses developed.

Metabolic Disease in Breeding Horses

So is there a parallel between the metabolic issues known to cause problems in human reproduction and those in horses? It's possible.

Veterinarians do know that metabolic syndrome in mares can negatively affect the insulin-like growth factors, which are key to follicle selection and dominance. These negative effects might ultimately impair ovarian function.

Additionally, he said, obesity has been shown to prolong estrous duration—leading to fewer usable breeding cycles each year—and ovulatory activity during the nonbreeding season. And while research has shown that a drug called metformin can help obese horses shed pounds and body systems return closer to normal, Morresey said it has not been uniformly effective in helping resolve obese mares' reproductive issues.

Another concern, he said, is gestational insulin resistance. All mares—even otherwise healthy ones—become insulin resistant during late pregnancy (generally after about 270 days, he said) and exhibit exaggerated insulin and glycemic responses when consuming high starch feeds compared to mares consuming a fat- and fiber-based diet. If already insulin resistant going into pregnancy the negative effects might well be amplified, he said.

It still isn't clear what impact metabolic disease has on stallion reproduction, Morresey said.

Managing Metabolic Disease in Breeding Horses

So how should owners and veterinarians manage breeding horses with or suspected of having metabolic disease?

First, Morresey said, make a good diagnosis: "Metabolic syndrome is a syndrome that's difficult to diagnose, a syndrome with profound effects, and a syndrome that's difficult to distinguish (from other endocrine disorders)."

He cautioned that while there are diagnostic options available, research has shown that none of the antemortem diagnostic tests are completely reliable. In fact, he said, they might be less reliable than clinical impression.

His advice: "Don't let a diagnostic test get in the way of a good diagnosis." He encouraged practitioners to rely on clinical impressions during the diagnostic procedures. He suggested they watch for abnormal fat distribution on the horse's body—especially in less common, but no less important, areas including in front of the mammary glands and sheath—and for hoof ridges as tell-tale signs of metabolic disease. He encouraged attendees to pay close attention to fat in the nuchal ligament (in the neck, which forms the "cresty neck"), which has been proven to have higher levels of pro-inflammatory cytokines (mediators of inflammation) than fat in other locations.

After making a diagnosis, Morresey said, it's time to implement a multifaceted treatment:

Dietary Control—"Acceptance is the first step to recovery," he said, so veterinarians might need to help an owner accept that his or her horse needs to lose some weight. Once everyone is on the same page, remove concentrates from the horse's diet, limit pasture access (either through a "pony prison" drylot or using a grazing muzzle), provide hay at a rate of 1 to 1.25% body weight, and divide the horse's rations into several smaller feedings throughout the day.

Exercise—Exercise can be a challenging step to accomplish with a horse that is foot sore or reluctant to move, but it's often in the horse's best interest to get them moving. Morresey said there's conflicting study results on whether exercise improves insulin sensitivity—one study suggested it didn't, while another suggested it did. Morresey's advice? "Just keep the horse moving."

Pharmacologic Aids—Some horses might require medical assistance to help get them back on track, Morresey said. There are several different drugs that might prove beneficial. Metformin has been successful in helping horses in clinical practice, specifically by decreasing glucose production by the liver, increasing peripheral absorption of glucose by cells, and decreasing glucose absorption from the gut (these are the proven human mechanisms, he said); however, he noted that the drug hasn't always performed well in equine research settings. Another medication, L-thyroxine, has also been shown to increase insulin sensitivity in horses; Morresey noted that if he had to choose a sole medication for a metabolic horse as a result of client constraints, he'd likely select L-thyroxine. Additionally, he noted, if a mare is suspected of having or has Cushing's disease, he recommended putting her on pergolide, despite the fact that one of the drug's side effects is agalactia (the absence of colostrum and milk production).
"You can get colostrum from another source," he said, adding that in clinical practice, he and others have successfully "maintained pregnant mares on pergolide throughout gestation when dictated by the health of the mare without detriment to foal delivery or adequate milk production."

Questions Remain

While researchers are making strides to understand metabolic disease's effects on reproduction, much is still a mystery. However, there is evidence to suggests that metabolic diseases could negatively affect equine reproduction.

About the Author

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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